Update of the underlying Firefox (gecko) code to v19. This has a number of consequences:

Add-ons and themes may need to be updated since the UI code has changed.

I'm aware that this may be inconvenient for users with a lot of add-ons installed, but the core engine changes are of too great importance not to push forward. Add-ons compatible with the v15 code base may not work properly or at all on the new release because of the far-reaching changes to the rendering engine, CSS and HTML5 implementations and evolving standards. It is strongly recommended not to force compatibility (changing files in xpi or telling pale Moon not to check extension compatibility) but instead to find updates for your add-ons, or, if no "Firefox 19" compatible versions are available, look for alternatives with the same functionality.

HTML5-implementation is more complete.

More elements of the still-evolving HTML5 standard have been implemented, giving the browser more functionality and features for web developers to use.

A number of CSS statements have their prefix removed (-moz*).

CSS statements that were considered provisional or experimental have been finalized, and the syntax with the provisional prefix "-moz-" has been replaced with the finalized CSS syntax. For example, "-moz-transition:" should no longer be used, but instead "transition:".

Javascript now uses the IonMonkey engine by default, which is a new (faster) engine.

IonMonkey has been some time in the making, and is included in Pale Moon 19.0 and enabled by default. It replaces the previous engine ("JägerMonkey"), but both engines are still included in thebrowser. If required for some reason, e.g. if a breaking issue pops up with the IonMonkey engine, you can switch engines by setting a preference and restarting the browser. (javascript.options.ion.content)

Improvements to the layout and rendering engines

The most important improvements in the gecko v19 core are improvements to the layout and rendering engine. This is all under-the-hood, to improve page display speed and the use of your GPU if possible.

If you are using a language pack, you need to update it to the latest version!

Pale Moon normally refuses to run an older language pack but if you have disabled version checking in the browser, you will end up with a non-working language pack when updating that will prevent the browser from starting. If this happened, you can start Pale Moon in safe mode and download the proper language pack for v19.0

Update of the browser style. Main browser controls and the padlock look slightly different.

This style change was mainly prompted by a strange glitch in the back/forward button, making it not work as it should with the Pale Moon theme and layout. The navigation controls have been slightly re-styled, specifically for disabled controls. To streamline the overall theme, the older "striped" padlock has been replaced with a look similar to the rest of the controls; Also a change in color from yellow to blue for regular HTTPS to stay in line with the rest of the identity panel.

Several Pale Moon specific improvements to the rendering engine, noticeable in general use and certain benchmarks, to prevent browser stalls or high CPU usage on certain pages.

This includes, among other things, improvements to scaled images (e.g. IE Chalkboard test) and certain types of gradients that are off-loaded to the GPU for rendering. Specific attention has been paid to mitigate a number of regressions present in current releases of Firefox, that would in fact cause the browser to stall or become completely unresponsive; most of these issues and solutions to them are Windows-specific.

The builds no longer use PGO (Profile Guided Optimization) but are globally speed-optimized.

PGO has shown to be less effective in VS2012 on the Mozilla code, and the workarounds/tradeoffs to circumvent compiler issues can, as a result, be removed, resulting in code that is optimized better overall. This decision is in line with the move away from synthetic benchmarks and towards overall browser smoothness and performance in daily use.

Main points where this release differs from Firefox 19:

The in-browser PDF viewer is not enabled

Pale Moon's approach has always been that the browser should remain a browser, not spread thin over multiple disciplines or opening up security issues by integrating and enabling potentially exploitable code. The PDF viewer code for in-line viewing of PDF documents is present but disabled by default. It is provided as an emergency measure only (just like in v15) and you are at all times strongly urged to use a standalone, external viewer for PDF files.

The PDF viewer has been available in Firefox Aurora and Beta builds for a number of months, and Mozilla has finally pushed it through to the release channel, but this is despite the fact Mozilla has acknowledged the existence of “several UX/rendering/stability concerns”.

You can still enable this code, at your own risk, by setting pdfjs.disabled to false and restarting the browser. Be aware that you will be missing more advanced features like form filling that you may find in a proper external viewer, and I will not provide official support for this PDF viewer or the potential rendering, stability or security issues there are involved in using this Javascript-based, in-browser viewer.

Several user interface items are present that were removed from Firefox

In Firefox, switching Tabs to the bottom once they have been placed on top has to be done through about:config - Pale Moon retains the context menu option in both directions

In Firefox, there no longer is a "Send Page" or "Send Link" context menu item to quickly open a new e-mail message in the configured mail application with a link to the current page or pointed link. Pale Moon retains this feature.

In Firefox, the "Safe mode" startup dialog has one recovery function next to safe mode: reset the entire profile. Pale Moon retains several choices for troubleshooting and resetting parts of the profile, and completely resetting your profile is done from about:support instead.

Release notes
19.0.1 (2013-02-24)
A minor update to address a few issues with the initial major release:

Fix for bookmarks giving an "XML parsing error" when set to "load in the sidebar"
Fix for a double padlock display if a secure site would not supply a favicon
Redone the mixed content https padlock image in 32bpp to prevent potential UI rendering issues
Fixed a setting so no unnecessary code walking is done for the otherwise disabled accessibility features
Fix (inherent) for add-ons and themes being marked as incompatible in Pale Moon x64 when they have a minimum version of 19.0